1. Field of the Invention
The present invention relates to a method for modifying the surfaces of materials to impart desired characteristics thereto.
2. Prior Art
In selecting materials for constructing articles, the artisan is often faced with a perplexing dilemma. A specific material may meet most of the requirements of the proposed application, such as strength, weight, density, structure, machinability, electromagnetic properties, etc.; however, its surface characteristics may render it unsuitable. For example, studies have shown that the surgical implantation of ocular implants such as intraocular lenses (IOL), etc., results in the loss of significant corneal endothelial tissue unless great care is taken to ensure a lack of contact between the device and the endothelium. Most ocular implants are constructed of hydrophobic polymethyl methacrylate (PMMA) polymers because of their superior optical qualities, resistance to biodegradation, etc. It has been found, however, that PMMA surfaces adhere to endothelial cells upon even casual contact and that separation of the surface therefrom results in a tearing away of the endothelial tissue adhered to the polymer surface. Similar adhesive interactions with other ocular tissues, i.e., the iris, can also cause adverse tissue damage. Other hydrophobic polymers which are used or have been proposed for use in ocular implants (i.e., polypropylene, polyvinylidene fluoride, polycarbonate, polysiloxane) also can adhere to ocular tissue and thereby promote tissue damage.
It is well documented in the prior art that a significant disadvantage inherent in PMMA IOLs resides in the potential for long-term abrasive interactions with sensitive tissues such as the iris, ciliary sulcus, etc., and that even brief contact between the corneal endothelium and hydrophobic polymer surfaces, i.e., PMMA, can result in extensive damage to the endothelium. See Katz et al, Trans. Am. Acad. Ophth., Vol. 83, p. 204-212 (1977).
Since it is extremely difficult to avoid any contact between implant surfaces and sensitive tissue surfaces such as ocular tissue during surgical procedures, efforts have been undertaken in ocular surgery to modify ocular implant surfaces, i.e., PMMA, to reduce the tendency thereof to adhere to and damage the corneal endothelium.
Ocular implant surfaces have been coated with various hydrophilic polymer solutions or temporary soluble coatings such as methylcellulose, polyvinylpyrrolidone, etc., to reduce the degree of adhesion between the implant surfaces and endothelial tissue cells. While offering some temporary protection, these methods have not proven entirely satisfactory, since such coatings complicate surgery, do not adhere adequately to the implant surfaces, become dislodged or deteriorate after implantation, dissolve away rapidly during or soon after surgery, or may produce adverse post-operative complications. Moreover, it is difficult to control the thicknesses and uniformity of such coatings.
Various surface modification techniques have already been proposed, and some of them are in use. For instance, surface grafting of hydrophilic monomers onto hydrophobic polymers has been achieved by photo-induced [Oster et al, J. Polym. Sci., Vol. 26, p. 233 (1957); Oster et al, Ibid., Vol. 34, p. 67 (1959); Tazuke et al, J. Polym. Sci., Polym. Lett. Edn. 16, p. 497 (1978); and Ogiwara et al, J. Appl. Polym. Sci., Vol. 23, p. 2397 (1979)] and radiation-induced polymerization [Jansen et al, J. Biomed. Mater. Res., Vol. 19, p. 1085 (1985); Boffa et al, J. Biomed. Mater. Res., Vol. 11, p. 317 (1977)]. However, all these methods suffer major limitations, i.e., grafting reactions are not confined to the outer substrate surface layers; grafting reaction time is too long; the coatings obtained are generally only physically absorbed on the substrate surface; and, finally, because of the relatively high penetration power of the radiation required for grafting, permanent chemical and structural changes such as cross-linking and degradation are commonly encountered [Mukherjee et al, J. Macromol. Sci. -Rev. Macromol. Chem. Phys., Vol. C26(3), p. 475 (1986)].
In addition to polymeric ocular implants, there are also a wide variety of metallic, ceramic and polymeric medical instruments, devices and implants which could be beneficially surface modified to yield non-adherent tissue-protective and more blood-compatible hydrophilic polymer grafted surfaces. Improved methods and materials for hydrophilic polymer surface modification of various polymeric instruments, devices, etc., have been set forth in copending application Ser. No. 07/592,482, filed Oct. 5, 1990, now U.S. Pat. No. 5,100,689. However, certain polymer substrates, i.e., fluorocarbon polymers, and especially metal and ceramic substrates, are extremely resistant to effective grafting of uniform, highly adherent surface modifications by the major methods available, i.e., gamma radiation polymerization grafting and glow discharge plasma polymerization.
Glow discharge plasma (GDP) has been extensively studied for surface modification of biomedical polymers (Yasuda, in Plasma Polymerization, Academic Press, Inc. (1985); Kim et al, CRC Crit. Rev. in Biocomp., Vol. 1, p. 229 (1985); Ratner et al, in Trans. 2nd World Cong. Biomaterials, Washington, D.C. (1984)). GDP may be achieved most commonly by radio frequency induction, or by DC discharge or microwave methods and has been used in two primary ways: 1) surface etching and/or oxidation by plasma treatment, and 2) plasma thin film polymerization and deposition. GDP induced by an inductively coupled radio frequency current (RF-GDP) is a high-energy state of ionized gases formed by passing gas or vapor molecules through a high-energy field. The resulting activated species possess energy to chemically alter the surface of a substrate placed in the GDP by generating activated surface species such as radicals or ion radicals. When exposed to air, these radicals or other activated sites can also combine with oxygen to form sites for further chemical reaction and polymerization with various vinyl monomers. Furthermore, monomers present in the plasma may be activated and graft polymerized to activated sites on the substrate. Under GDP conditions, even relatively unreactive compounds such as benzene, toluene, perfluoropropane, etc., which are not vinyl monomers may also be sufficiently activated to enable polymer-forming reactions.
Observations indicate that when hydrophobic polymers, such as FEP (Teflon), PC (polycarbonate), PMMA (polymethylmethacrylate), PDMSO (polydimethylsiloxane), PP (polypropylene), etc., are placed in a plasma even at relatively low power and for short exposure times and contact traces of oxygen, they become more hydrophilic due to surface oxidation. It has also been demonstrated that many polymers can be reduced or oxidized depending upon GDP conditions, thus altering their surface properties (Clark et al, J. Polym. Sci., Polym. Chem. Edn. 21p. 837 (1983).
Plasma treatment can cause chain scission, ablation, cross-linking, oxidation and other reactions to a depth of 50-100 .ANG. or more depending on the substrate and experimental conditions [Wu et al, in Polymer Interphase and Adhesion, Chap. 9, p. 298, Marcel Dekker, New York (1982)].
The case of gamma polymerization alone has also been extensively studied for surface grafting of hydrophobic polymers [Boffa, supra; Hegazy et al, J. Appl. Polym. Sci., Vol. 26, p. 3117 (1981); Mukherjee et al, J. Appl. Polym. Sci., Vol. 30, p. 2643 (1985); Hoffman et al, Arch Phys. Chem., Vol. 22, p. 362 (1983)]; yet this method, by itself, is not always satisfactory. In addition to the problems mentioned earlier, gamma irradiation of substrate and monomer causes solution polymerization as well as grafting onto substrate. Grafting is dependent on the prevalence of excited surface species such as radicals generated by gamma radiation, which in turn is dependent upon the energy required to form such activated species in a particular substrate. Therefore, substrates with high activation energies for radical formation relative to monomer solutions do not easily graft by gamma polymerization before extensive solution polymerization and gelation occurs, making sample removal and washing impractical.
An improved and efficient method for the gamma-irradiation induced graft polymerization coating of the surfaces of ocular implants constructed of polymethylmethacrylate and other ocular implant polymers is disclosed in U.S. Pat. No. 4,806,382.
Although some polymeric substrates may be surface modified under certain conditions of gamma radiation graft polymerization, metallic and ceramic substrates pose extreme difficulties because low gamma radiation energy and the impermeability of such materials to monomeric molecules make such surface polymerization grafting impractical, leading to non-uniform, non-adherent structures.
The present invention comprises a novel surface grafting technique which overcomes all of the above problems. The technique is based on a combination of glow discharge plasma pretreatment of the solid substrate, followed by low dose gamma radiation or electron beam radiation initiated grafting in the presence of polymerizable monomer(s).
Although this combination of GDP and gamma or electron beam radiation graft polymerization represents a generally improved technique for polymer substrates, it is a uniquely effective process, heretofore unavailable, for the production of polymeric surface graft modifications of metal and ceramic substrates.
Only a few attempts have been reported in literature which employed plasma treatment combined with other treatments in order to induce grafting [Bamford et al, Polymer., Vol. 2, p. 277 (1961); Bazkin et al, J. Bioeng., Vol. 2, p. 527 (1978)]. Generally, these methods are based on decomposition of surface peroxides to radicals by heat treatment which initiates graft polymerization. However, heating up to 135.degree. C. was sometimes required [Bamford, supra]. This temperature is beyond the glass transition temperature of a large number of polymers. Therefore, such a process cannot be used for surface grafting, especially for biomedical devices such as acrylic intraocular lenses, where shape and dimensional stability are very important. In 1971, Bradley and Fales reported another process based on the plasma-induced graft copolymerization of acrylic acid onto synthetic fibers [Bradley et al, Chemtech., p. 232, April 1984]. Grafting was induced by the surface radicals present on the plasma treated fibers. The grafting yields were generally relatively low and no grafting of other monomers was attempted. In contrast, the combined use of GDP surface modification with gamma graft surface modification represents a novel method for the preparation of improved polymer graft surfaces on polymers, metals and ceramics.
It is an object of the present invention to provide a generally applicable method for modifying the surface characteristics of a wide variety of materials not subject to the above-noted problems and disadvantages.
It is a further object of the invention to provide novel articles having modified surfaces, thereby enabling their efficient use in applications for which they were not previously suited.